It has nothing to do with determinism. It's the difference between nearly perfectly but not quite perfectly translating between rigorously specified formal languages and translating an ambiguous natural language specification into a formal one.
The first is a purely mechanical process, the second is not and requires thousands of decisions that can go either way.
The difference is that a human is that a human can reason about their code changes to a much higher degree than an AI can. If you don't think this is true and you think we're working with AGI, why would you bother architecting anything all or building in any guard rails. Why not just feed the AI the text of the contract your working from and let it rip.
You give way too much credit to the average mid level ticket taker. And again, why do I care how the code does it as long as it meets the functional and none functional requirements?
The first is a purely mechanical process, the second is not and requires thousands of decisions that can go either way.